Draft new Additional Document Constraints

From Section 4 of the hub document, titled Additional Document Constraints

  In addition to the UBL 2.1 document constraints formally expressed by
  the schemas in Section 3, "UBL 2.1 Schemas", UBL mandates several
  other rules governing conformant UBL 2.1 instances that cannot be
  expressed using W3C Schema.These additional UBL document rules,
  addressing instance validation, character encoding, and empty elements,
  are specified below.